Controller edit method, modify object data before commit

Question

When a user saves a form, I want to do the following check before saving

if header2 is null
header2 = header1

I don't want to add validation and stop the save action, because header2 is only needed in rare occasions. I'm not sure how to have the update method in the controller do this. The command to commit the update is

if @entry.update_attributes(params[:entry])

So should I be modifying the params[:entry][:header2] field? I tried to do this with the following code but it isn't updating

params[:entry][:header2] = params[:entry][:header1]

Any help would be appreciated. I'm starting to feel I should handle this on the client side with javascript instead.

--EDIT: Added to original Post Below--

I have tried coderama's suggestions below and neither work for me

  validate :data_present

  def data_present
    self.header2 = self.header1 if self.header2 == nil
  end

and

  def update
    @entry = Entry.find(params[:id])
    params[:entry][:header2] = params[:entry][:header1] unless params[:entry][:header2].present?    

    respond_to do |format|
      if @entry.update_attributes(params[:entry])....

SQL

mysql> select header1, header2 from entries where title = "new";
+---------+---------+
| header1 | header2 |
+---------+---------+
| Blah    | NULL    |
+---------+---------+
1 row in set (0.00 sec)

I want be able to open in edit mode, change nothing (field is already null) and hit save and have this code changed. Have also tried for making new entry records and it doesn't work there either.

Answer 1

Addressing your own options, these could be possible solutions which I have both used successfully depending on my circumstances.

Option 1 - Model Validation

Create a custom validation or a validation based on a condition. E.g.

class Body < ActiveRecord::Base
  validates_presence_of :hair, :unless => lambda { |o| o.age > 80 }

  validate :multiple_heads

  def multiple_heads
    self.head = "No head" if self.head == nil
  end
end

Option 2 - Modify the hash in the controller

Your second suggestion will also work. E.g.

params[:entry][:header2] = params[:entry][:header1] unless params[:entry][:header2].present?

Answer 2

Here you go, along with the specs to prove

/app/models/model_name.rb

class ModelName < ActiveRecord::Base

  attr_accessor :header_1, :header_2
  before_validation :header_check

  def header_check
    return if self.header_2.blank?
    self.header_1 = self.header_2 if self.header_1.blank?
  end

end

And the spec: /spec/models/model_name_spec.rb

require 'spec_helper'

describe ModelName do

  before(:each) do
    @model_name = ModelName.new
    @header_one_text = "HeaderOneText"
    @header_two_text = "HeaderTwoText"
  end
  it "should should set header 2 if header 1 is blank" do
    @model_name.header_2 = @header_two_text
    @model_name.valid?
    @model_name.header_1.should == @header_two_text
  end

  it "should leave header 1 if both are set" do
    @model_name.header_1 = @header_one_text
    @model_name.header_2 = @header_two_text
    @model_name.valid?
    @model_name.header_1.should == @header_one_text
  end

end